1. Field of the Invention
The present invention relates to a multicarrier communication method and system using a balanced transmission line such as a power line, and a communication apparatus incorporated therein.
2. Description of the Related Art
With the rapid evolution of intelligent home appliances, attention has been directed to power-line communications (hereinafter simply called a “PLC”) that allow existing power lines to be used as transmission lines, without wiring new cables, in order to control the intelligent home appliances in each household.
In a wideband PLC system having expectation of its further development in the near future, a multicarrier system such as OFDM (Orthogonal Frequency Division Multiplexing) is predominantly employed as a PLC-modulating method. According to the multicarrier system, a so-called channel estimation method (a channel-evaluating method) has been developed to realize high-speed transmission. The channel estimation method sets a modulation method for each carrier to use only carriers having a smaller bit error rate, not carriers having a greater one.
The following discusses the prior art with reference to FIG. 8. FIG. 8 is a descriptive illustration showing a prior art channel estimation method. In FIG. 8, the horizontal axis denotes a frequency axis illustrating a position of each carrier, while the vertical axis denotes an SN-ratio of each of the carriers between transmission and reception. An SN-ratio envelope 40 in FIG. 8 interconnects SN-ratios of the carriers. In the SN-ratio envelope 40, a greater distance from the horizontal axis provides a better SN-ratio of each corresponding carrier.
In FIG. 8, 64QAM-threshold 51 denotes a minimum SN-ratio required to employ a 64QAM-modulation method (QAM is an abbreviation of quadrature amplitude modulation). 16QAM-threshold 52 denotes a minimum SN-ratio required to employ a 16QAM-modulation method. QPSK-threshold 53 denotes a minimum SN-ratio required to employ a QPSK-modulation method (a quadrature phase modulation method). BPSK-threshold 54 denotes a minimum SN-ratio required to employ a BPSK-modulation method (a binary phase shift keying modulation method).
In each of the above methods, the addition of proper error-correcting bits makes it feasible to correct data errors that have occurred in transmission lines, when each SN-ratio is equal or greater than its predetermined threshold. Each of the thresholds according to the modulation methods usually has an SN-ratio difference of some 6 dB.
The modulation methods have multivalues rendered greater in the order of the 64QAM-, 16QAM-, QPSK-, and BPSK-modulation methods. As a result, information transmittable per unit time is made greater in amount in the same order. At the same time, SN-ratios made greater in the same order are required.
The prior art channel estimation method selects a modulation method having a greater multivalue permitted by an SN-ratio of each of the carriers, and consequently transmits information more efficiently when viewed as a whole.
For example, each of carriers C1, C2, and C3 in FIG. 8 has an SN-ratio equal to or greater than QPSK-threshold 53 but less than 16QAM-threshold 52. Assuming that each of the carriers C1, C2, and C3 employs the 16QAM-modulation method, there is an increased possibility that non-correctable errors in data might occur because of an insufficient SN-ratio of each of these carriers. Accordingly, each of the above carriers selects the QPSK-modulation method in which its SN-ratio is sufficient. Carrier C4 has an SN-ratio greater than the 16QAM-threshold 52, but less than the 64QAM-threshold 51, and accordingly selects the 16QAM-modulation method. Similarly, carriers C31 to C34 select the 64QAM-modulation method.
There is another prior art of realizing a high-efficient multicarrier transmission method, which is now described below with reference to FIG. 8.
As discussed above, the carriers C1, C2, and C3 in FIG. 8 select the QPSK-modulation method, while the carrier C4 selects the 16QAM-modulation method. Each of the carriers C2, C3 has an SN-ratio considerably greater than the QPSK-threshold 53. This means that the carriers C2, C3 can be reduced in transmission power to a degree where their SN-ratios slightly exceed the QPSK-threshold 53. An amount of the reduction in transmission power may be imparted to the carrier C4 in order to increase the transmission power of the carrier C4 by a corresponding amount. As a result, the carrier C4 has an SN-ratio greater than the 64QAM-threshold 51, and is possible to select the 64QAM-modulation method having a greater multivalue.
In short, the transmission carrier power control method is an art of transmitting information with improved efficiency by controlling the transmission power of each of the carriers under the condition that the entire transmission power is constant.
The prior art channel estimation method and transmission carrier power control method are characterized in that the carriers are selected in accordance with only their SN-ratios. However, the PLC has particularity in which indoor power lines are used as transmission lines, and consequently has a problem of transmission power leakage or unwanted radiated emission caused by the above particularity. This is a more serious issue to a high-frequency band PLC that is expected to evolve in the future. Therefore, in the multicarrier system, the carriers must be selected in light of the suppression of the unwanted radiated emission as well as the SN-ratios.
[Patent reference No. 1]: published Japanese Patent Application Laid-Open No. (HEI) 3-265314
[Patent reference No. 2]: published Japanese Patent Application Laid-Open No. 2000-332723
[Patent reference No. 3]: published Japanese Patent Application Laid-Open No. 2001-274748
[Patent reference No. 4]: published Japanese Patent Application Laid-Open No. 2001-320306
[Patent reference No. 5]: published Japanese Patent Application Laid-Open No. 2002-280935